Pleistocene fluvial terraces from northern France (Seine,Yonne, Somme): synthesis,and new results from interglacial deposits

The Seine, Yonne and Somme are the main rivers draining northwestern France and flowing into the Channel. During Pleistocene cold stages they were tributaries of the “Fleuve Manche”. They are characterised by well-developed stepped terrace systems showing up to 10 incision steps for the last 1 Ma. After 15 years of research and the synthesis of stratigraphical, sedimentological, bioclimatical data and numerical dating, these terrace systems are interpreted as the response of the fluvial environments to climatic cyclicity, superimposed on a background of slow tectonic uplift. The comparison of these three terrace systems shows that the incision budget is similar within the studied area (about 55–65 m/1 Ma), implying relative homogeneity of uplift. In the main part of the area, beyond marine influence, the analysis of each stepped alluvial formation shows regular sediment balance generally composed of coarse periglacial gravels (pleniglacial), covered by fine-grained sediments locally overlain by calcareous tufa (interglacial). However, in the Lower Seine valley, close to the present coast, the low terrace includes two estuarine interglacial units interbedded within periglacial gravel units. Nevertheless, until recently evidence of full interglacial conditions remained elusive in fluvial contexts, especially in the Somme. To address these issues a new research programme has been launched on tufa deposits that are the best candidate to register the climatic optimum. Initial results from la Celle (Seine), Saint-Acheul and Caours (Somme) allow palaeoenvironmental reconstructions of MIS 11 and 5e, and thus confirm the importance of calcareous tufa to define a reference record of Pleistocene Interglacials for northern France. Moreover, it has been previously demonstrated that the main incision leading to terrace formation can be attributed to the transition between interglacial and glacial (during early glacial phases). The new results from interglacial records have important implications concerning the precise occurrence of the major incision phases within the climatic cycle as they provide the starting point immediately preceding downcutting.     

The Holocene tufa at Daours (Somme Valley,northern France): Malacological succession and palaeohydrological implications

One of the thickest and most extensive tufa deposits in northern France occurs at Daours, about 8 km upstream of Amiens at the confluence of the River Somme and its tributary the Hallue. It covers an area of about 80 ha and reaches 10 m above the level of the present valley, probably owing to the construction of a tufa barrage, which caused periodic damming of the valley. The molluscan succession from a 4‐m section through the Daours tufa provides an unusually detailed record of environmental change during the Holocene. The faunal record, supported by four radiocarbon dates, indicates that the tufa sequence at this location does not extend back to the early Holocene but began c. 7340 cal. a BP and ceased to form shortly after c. 4938 cal. a BP, consistent with the notion of a NW European ‘late Holocene tufa decline’. The molluscan record is divisible into six zones that define three aquatic phases separated by three terrestrial episodes, when the tufa surface dried out sufficiently to allow colonization by dry‐ground species, including some shade‐demanding elements. These events are also reflected by differences in the lithology of the tufa. The earliest aquatic phase at the base of the sequence represents the most stable and permanent water‐body. The two subsequent aquatic phases appear to represent smaller, more ephemeral, water‐bodies surrounded by marsh. The land snail assemblages show ecological changes within each terrestrial episode, as well as faunal differences between them. These events, which reflect changes in the local hydrology, were short‐lived, each lasting for only a few hundred years. The tufa at Daours has also yielded flint artefacts and pottery, but it seems unlikely that the dynamic environmental record of the site results from anthropogenic activity.     

A sedimentary facies model for stepped,fluvial tufa systems in the Iberian Range (Spain): the Quaternary Piedra and Mesa valleys

Stratigraphic and sedimentological analyses of the Quaternary tufa and associated deposits in the Piedra and Mesa river valleys allowed a number of stages of their sedimentary evolution to be characterized, and a depositional sedimentary model for this north‐central sector of the Iberian Range (Spain) to be established. The proposed sedimentary facies model may explain tufa arrangements in other medium to high gradient, stepped, fluvial tufa systems with narrow transverse profiles occurring in temperate, semi‐arid areas, in both recent and past scenarios. There are several tufa deposits within the Piedra and Mesa river valleys that, over a maximum thickness of about 90 m, record one or more stages of tufa deposition produced following the fluvial incision of the bedrock or previous tufa deposits. Each depositional stage begins with coarse detrital sedimentation. Six fundamental, vertical sequences of tufa facies with small amounts of detrital material reveal the sedimentary processes that occurred in different fluvial environments: channel areas with: (i) free‐flowing water; (ii) barrages and/or cascades; and (iii) dammed water and palustrine floodplains. The proposed sedimentary model involves narrow, stepped, fluvial valleys in which tufa cascades were common. Alternating intervals of bryophyte and stromatolite facies commonly formed at some cascades. Many of these represented barrage‐cascade structures that consisted of phytoclast rudstones, thick phytoherms of mosses and associated stromatolites, and curtain‐shaped phytoherms of stems. Upstream of these structures, dammed areas with bioclastic sands and silts developed and palustrine vegetation grew. The channel stretches between barrages and/or cascades were loci for extensive stromatolite growth in fast flowing water. The palustrine floodplain was home to pools and drainage channels. The model also explains the growth of some barrages in the River Piedra that surpassed the height of the divide, with the diffluence of the main channel into a secondary course forming other tufa deposits in the area. The distribution and abundance of certain types of tufa facies in fluvial basins may be an indicator of differences in their gradients. The facies studied in this work suggest that the gradient of the ancient River Piedra was steeper than that of the ancient River Mesa. Assuming similar scenarios for climate and hydrology, the depositional settings mentioned above and their dimensions would have been determined mainly by the gradient and width of the associated river valleys. This sedimentary model may also be useful for inferring variations in other river basin slopes, as well as accounting for the presence of tufas in areas that normally have no permanent water input.     

A late Middle Pleistocene temperate–periglacial–temperate sequence (Oxygen Isotope Stages 7–5e) near Marsworth, Buckinghamshire, UK

River-channel and colluvial deposits, near Marsworth, Buckinghamshire, record a temperate-periglacial-temperate sequence during the late Middle Pleistocene. The deposits of a lower channel contain tufa clasts bearing leaf impressions that include Acer sp., and Sorbus aucuparia and containing temperate arboreal pollen attributed to ash-dominated woodland. The tufa probably formed at the mouth of a limestone spring before being redeposited in a small river whose deposits contain plant remains, Mollusca, Coleoptera, Ostracoda and vertebrate bones of temperate affinities. The sediments, sedimentary structures and limited biological remains above the Lower Channel deposits indicate that fluvial deposition preceded climatic cooling into periglacial conditions. Fluvial deposition recurred during a later temperate episode, as shown by the mammalian bone assemblage in stratigraphically higher channel deposits. The Upper Channel deposits are confidently attributed to Oxygen Isotope Sub-Stage 5e (Ipswichian) on the basis of their vertebrate remains. However, the age of the Lower Channel deposits is less clear. The mammalian and coleopteran remains in the Lower Channel strongly suggest correlation with Oxygen Isotope Stage 7 on the basis of their similarities to other sites whose stratigraphy is better known and the clear difference of the Lower Channel assemblage from well-established faunas of Ipswichian or any other age. By contrast, U–Th dating of the tufa clasts suggests an age post 160 ka BP, while Aile/Ile ratios on Mollusca point to an Ipswichian age and younger. Four ways of interpreting this age discrepancy are considered, the preferred one correlating the Lower Channel deposits with Oxygen Isotope Stage 7.     

Carbonate tufas as archives of climate and sedimentary dynamic in volcanic settings,examples from Gran Canaria (Spain)

Three Holocene tufas from Gran Canaria volcanic island were studied with the aim of deciphering their sedimentary evolution through space and time. Las Temisas tufa (south-eastern arid part of the island) is dominantly composed of oncoids, intraclasts, phytoclasts, coated stems, minor thin stromatolites, and a high amount of siliciclastics. It was deposited in a fluvial system with variable flow velocities and palustrine conditions areas, which alternated with high energy events. Azuaje tufa (northern humid part of the island) is composed of coated stems, stromatolites, oncoids and phytoclasts, with relatively low amounts of siliciclastics, suggesting slow-flowing and palustrine conditions and a relatively low incidence of (high energy) floodings. Los Berrazales tufa (north-west of Gran Ganaria, the most humid one), is mainly composed of coated stems and crystalline crusts, formed in a laminar flow regime. Dominant clastic sedimentation in Las Temisas and high calcite growth rates in Los Berrazales led to a poor development of stromatolites in comparison with Azuaje. Las Temisas and Azuaje deposits have similar upward evolution with decreasing trend in siliciclastics and increasing trend in carbonates. However, Las Temisas has higher siliciclastic and lower phytoclastic contents suggesting a less vegetated area and more arid climate than in the other deposits. Additionally, tufas record local events common in volcanic terrains. Azuaje presents three units bounded by erosive discontinuities, which reveal significant erosion by enhanced runoff that could be caused by loss of vegetation due to wildfires related to volcanic eruptions at headwaters. Las Temisas record a possible interruption in sedimentation represented by aligned boulders due to rockfalls from the hillsides. These deposits formed from waters with similar chemistry providing to the carbonates their similar signals in δ¹³C–δ¹⁸O stable isotopes and ⁸⁷Sr/⁸⁶Sr ratios like that of the volcanic rocks. This work shows how, in volcanic areas, tufas are unique archives of the climate, vegetation and volcanic-related processes, because all imprint the sedimentary regime of tufa deposition.     

柴达木盆地诺木洪贝壳堤水体演变的介形类证据

柴达木盆地贝壳堤保存有大量大小混杂及两壳绞合的双壳类化石,是全球干旱区罕见的地貌景观,为青藏高原东北部中晚更新世的气候与环境演化提供了重要的区域地质记录.该记录在诺木洪西北约20 km出露,将其命名为诺木洪贝壳堤.目前诺木洪贝壳堤的沉积属性存在河湖之争,对该贝壳堤剖面开展系统的介形类分析,建立了8个生物组合带,除1带化...     

Confirmation of an interglacial age for the Condat tufa (Dordogne,France) from biostratigraphic and isotopic data

The Condat tufa is a localised deposit, reaching 8.2m in thickness, composed of freshwater carbonates deposited in a complex series of environments which include marsh, spring and stream facies. Five radiocarbon determinations had previously been obtained from different stratigraphic levels within the tufa and had given a consistent series, ranging from 12320 ± 175 BP to 31050 ± 1500BP. Although the youngest two dates, from near the surface of the tufa, had been rejected as being anomalously young, the older dates were regarded as indicating tufa formation during the last cold stage. Biostratigraphic analyses of the molluscan and ostracod faunas, presented here, demonstrate that the tufa formed under fully temperate conditions throughout. The occurrence of certain snails that are either locally extinct (e.g. Belgrandia marginata) or unknown living in western France (e.g. Daudebardia brevipes), together with the geomorphological setting, suggest that the tufa is of interglacial age. New radiocarbon determinations of the Condat tufa all proved to be >33 Ka BP and support this conclusion.     

The infill of a supradetachment(?) basin: the continental to shallow-marine Upper Permian succession in the Dolomites and Carnia (Italy)

In the Dolomites and Carnia (eastern Southern Alps), the Upper Permian succession is represented by red beds of Val Gardena Sandstone, grading upwards and eastwards into the evaporitic and carbonate deposits of the Bellerophon Formation.

An overall transgressive pattern is shown by the general trend of the depositional setting, which evolved from flashy alluvial fans, through multiple-channel bedload rivers, mixed-load sinuous rivers, terminal fans, coastal sabkha and evaporitic lagoon, to a shallow, low-gradient marine ramp. The inferred fluvial regime was subject to rapid and erratic fluctuations in discharge. Palaeosols are represented by calcic soils, and suggest a warm to hot, semi-arid or dry subhumid climate with strongly seasonal rainfall. Vertic features are associated with more inland alluvial complexes: they are missing in the terminal-fan deposits, suggesting greater aridity in lowland (coastal) areas.

The Bellerophon Fm. consists of two units: a lower evaportte-bearing unit, deposited in a barred basin, and an upper shallow-marine carbonate unit, laid down on a very low-energy, low-gradient ramp.

Five third-order sequences and the lower part of a sixth sequence, collectively showing a backstepping pattern, have been identified in the succession. Due to the presence of a very low-gradient ramp margin, and the consequent ineffectiveness of relative sea-level falls in producing large-scale erosion in coastal areas, it is suggested that, rather than eustatic changes, degradational episodes marking sequence boundaries in the red-bed succession reflect changes in the climate-modulated intrinsic variables of discharge and sediment supply, and/or tectonic uplift.

Base level rises in, red-bed sequences are recorded by upward change from thick channel-belt sandstone bodies with an often high connectedness ratio, to progressively thinner channel deposits, ranging from isolated ribbons to channel-belt sandstone bodies potentially reaching high width/thickness ratios, encased in a comparably greater volume of overbank fines. This trend is thought to reflect the change from a confined geomorphic setting, with a limited area of potential avulsion, to an unrestricted setting with rivers free to move extensively; in addition, it documents the transition from an inland fluvial system with laterally migrating perennial or semi-perennial channels, to an ephemeral network of randomly migrating and frequently avulsing small terminal-fan distributaries, through a drastic downstream decrease in channel depth and discharge.

Identification of key sequence-stratigraphic relationships within the red-bed succession was mostly aided by the presence of easily recognizable and regionally correlated marine and marginal marine bed packages, traceable landwards into alluvial deposits showing faint traces of tidal activity, interpreted as the equivalent of marine maximum-flooding sediments. They may grade upwards into progradational fluvial packages showing basinward increase in thickness.

The Upper Permian deposits of the Southern Alps are considered part of an Upper Permian-Lower Triassic, second-order, structurally controlled sequence. The location of the basin on a thickened, previously active crust, affected by thermal perturbance after the last stages of the Variscan orogeny, the relatively reduced thickness of the basin fill, and predominantly long, transverse drainage networks, mostly derived from the denuded Insubric footwall, all suggest that sedimentation took place in a supradetachment basin, with a major detachment fault located in the palaeo-Insubric belt.     

Palaeoenvironments of a complete Eemian sequence at Mommark, South Denmark: foraminifera, ostracods and stable isotopes

A new investigation of the coastal cliff section at Mommark in southern Denmark has revealed a complete Eemian interglacial sequence for the first time in the southwestern Baltic area. Environmental changes through the lacustrine and marine interglacial deposits are discussed on the basis of foraminiferal assemblages and stable isotope composition as well as ostracods. In general, the assemblages indicate relatively high temperatures throughout the Eemian, and the Lusitanian foraminiferal species Pseudoeponides falsobeccarii Rouvillois has been reported for the first time from the Eemian of northwest Europe. A floating chronology of the deposits is based on a previously published correlation of the local pollen stratigraphy with annually laminated sequences in northern Germany. An initial early Eemian lacustrine phase, with ostracodal indication of deposition in a large freshwater lake, lasted until c. 300 years after the beginning of the interglacial, i.e. to the transition between the regional pollen zones E2 and E3. After that, marine conditions persisted almost throughout the interglacial, and the Cyprina Clay was deposited. The foraminiferal and ostracodal assemblages indicate that relatively deep water prevailed in the area until c. 6000 years after the beginning of the interglacial. However, both the foraminiferal assemblages and the oxygen isotope results show that a trend from relatively high salinity to lower salinity conditions had begun already at about 4000 years. After c. 6000 years the fauna indicates a gradual change to shallower water and further reduction in salinity, the latter also being reflected by a general decrease in the oxygen isotope values. The marine deposition ended at c. 10 600 years after the beginning of the Eemian, i.e. within the topmost part of pollen zone E7. This was succeeded by a late Eemian and early Weichselian freshwater phase.     

西藏特提斯喜马拉雅海相白垩纪—古近纪生物地层与重大地质事件研究进展

地质历史时期曾发过许多对生命的演化进程造成过重大影响与制约的全球性地质事件,白垩纪—古近纪就是一个重大地质事件频发的时期。随着冈瓦纳大陆在中生代时期的解体,全球海陆格局发生了根本的变化,地球的表层和岩石圈层均发生了重大的改变,由此引发了构造运动空前活跃,发生过诸如大洋缺氧事件(OAE)、大洋富氧事件(CORBs)、白垩纪/古近纪(K/Pg)生物大灭绝事件、古新世—始新世极热(PETM)事件、印度-亚洲板块碰撞、新特提斯洋的演化及最终消亡等一系列的全球性重大地质事件。对这些重大地质事件的研究,有助于加深我们对古海洋、古地理、古环境的认识。尝试追踪和捕捉这些重大地质事件,恢复和重建古地理,其基础是建立精确的年代地层格架。西藏南部保存了中国最为完整的海相白垩纪—古近纪沉积,完整地记录了上述的全球性重大地质事件,通过对札达、岗巴、定日、亚东、江孜、萨嘎和吉隆等地区高分辨率浮游和底栖有孔虫、介形虫、钙质超微化石和放射虫生物地层学研究,可直接约束全球性重大地质事件发生的时间,并为重建新特提斯洋古海洋环境和古地理提供证据。此外,在重大地质环境突变期间生物的演化过程,也可为探明极端环境变化发生时期气候-环境-生物之间的协同演化关系提供证据。本文系统总结了课题组为主的近年来对藏南白垩纪—古近纪海相地层中微体古生物学的研究成果及重要进展,并对未来研究提出展望。     

Notes on fossil remains from the Early Cretaceous Lohan Cura Formation, Neuquén Province, Argentina

In this contribution, we describe an association of fossil remains collected in the locality of Cerro de Los Leones, approximately 8 km southwest of the town of Picún Leufú, Neuquén Province, Argentina, from different levels of the Albian Cullín Grande Member, Lohan Cura Formation. The fossils include pelecypodan (Neocorbicula dinosauriorum, Neocorbicula pehuenchensis, and genus indet.) and gastropodan (Physa wichmanni) molluscs, a mesoeucrocodylian, possible pterosaurs, theropods, and sauropods. The sedimentary environment at Cerro de los Leones is characterized by fluvial deposits corresponding to a sandy meandering fluvial system. The most fossiliferous levels (located in the lower part of the exposed sequence) were developed in a distal flood plain and flood basin areas. Sedimentological and paleopedological features suggest semi-arid conditions during the deposition of the Cullín Grande Member. Despite the fragmentary nature of the specimens described here, the new information provided from the Albian assemblages of the Lohan Cura Formation at the Cerro de Los Leones locality provides a broader panorama of the composition of terrestrial faunas during the Early Cretaceous in Patagonia.     

Estuaries of the past and present: A biofacies perspective

Estuaries are complex sedimentary and ecological systems, where controlling factors are variable largely depending on wave vs. tidal dominance and fluvial processes. Paleoenvironmental reconstruction of their ancient counterparts in the form of coastal valley deposits in the subsurface or outcrop requires a multidisciplinary approach. Microfossils can play an integral part in identifying estuarine subenvironments. Foraminifera can be abundant in modern estuaries and resemble characteristics of brackish ichnofaunal communities in featuring low species diversity, but high abundances of opportunistic species, different feeding strategies and common infaunal species. Whereas sediment distribution is highly controlled by energy regimes, foraminifera seem to respond to salinities and tidal exposure. Whereas individual taxa can widely range bathymetrically, the combination of certain taxa becomes diagnostic for estuarine environments. Fossil marginal marine assemblages are dominated by agglutinated species due to taphonomic loss of the calcareous component that is often dominant in modern estuaries. When comparisons between fossil and modern assemblages are undertaken it is advisable to compare with Recent subsurface or remnant assemblages for a more accurate basis of paleoenvironmental interpretation. More integrated research with detailed taphonomic observations is needed on ancient coastal valley deposits to find the paleoecological requirements of extinct taxa and their link to sedimentary facies and ichnofacies.     

Sedimentology and geochemistry of fluvio-lacustrine tufa deposits controlled by evaporite solution subsidence in the central Ebro Depression, NE Spain

The Urrea de Jalón tufa deposits constitute the 20‐ to 50‐m‐thick caprock (0·3 km²) of an isolated mesa. They disconformably overlie horizontal strata of the Tertiary Ebro Basin (NE Spain), which contains a thick succession of lacustrine gypsum and marls, followed by limestones, marls and, locally, fluvial sandstones and mudstones. The tufa deposits show a complex, large‐scale framework of basin‐like structures with centripetal dips that decrease progressively from the base to the top of the tufa succession, and beds that thicken towards the centre of the structure (cumulative wedge‐out systems). These geometries reveal that the tufa deposits were affected by differential synsedimentary subsidence. Distinct onlapping depressions reflect time migration of the subsiding areas. The studied carbonates are composed mostly of low‐Mg calcite, with minor quartz. Some samples have anomalously high contents of Fe, Mn and Ba that may exceed 1% (goethite, haematite and barite are present). Carbonate facies are: (a) macrophyte encrustation deposits; (b) bryophyte build‐ups; (c) oncolite and coated grain rudstones; (d) non‐concentric stromatolite‐like structures; (e) massive or bioturbated biomicrites; and (f) green and grey marls. Facies a and c show a great variety of microbial‐related forms. These facies can be arranged in dm‐ to 2‐m‐thick vertical associations representing: (i) fluvial–paludal sequences with bryophyte growths; (ii) pond‐influenced fluvial sequences; and (iii) lacustrine–palustrine sequences. The Urrea de Jalón tufa deposits formed in a fluvio‐lacustrine environment that received little alluvial sediment supply. Isotope compositions (δ¹³C and δ¹⁸O) reveal meteoric signatures and accord with such a hydrologically open system of fresh waters. The Fe, Mn and Ba contents suggest an additional supply of mineralized waters that could be related to springs. These would have been discharge points in the Ebro Depression of a regional aquifer of the Iberian Ranges. Rising groundwater caused the solution of the underlying evaporites and the synsedimentary subsidence of the tufa deposits.     

Assessment of the collapse mechanisms of tufa deposits

This research assesses the collapse mechanisms of tufa deposits. The city of Antalya, located on the Mediterranean coast of Turkey, has been settled on tufa deposits. By the end of 1980s, the area behind the tufa cliffs became the site of high-rise residential buildings. Some of these buildings have suffered from foundation instabilities, which have given rise to cracking and fissuring of the walls, and overall tilting. The collapsible behaviour upon loading and/or wetting of some tufa deposits has caused foundation settlement.

Voids, bonding of grains and grain properties of the collapsible tufa were investigated using optical and scanning electron microscopes. Our studies indicate that the collapse behaviour of tufa is affected mainly by depositional environment and microfabric. Laboratory tests have been performed in order to determine the index properties and the collapse potential of tufa samples. Young tufa deposits near the surface are weakly bonded with thin films of carbonate cement, giving rise to high void ratios. Analysis of geotechnical parameters has shown that the collapse potential of tufa is closely related to the natural void ratio and the amount of cement binding the grains.     

Holocene climate as reflected by a malacological sequence at Verrières, France

Though numerous analyses have been made of Holocene pollen sequences, they come from similar environmental contexts, mainly peat deposits. Land snails can provide good palaeoecological and palaeoclimatical data in different drier environmental settings. The Verrières deposits, located in the Seine Valley, southeast of Paris, provide rich and abundant malacofaunas. We compare the well–defined local biostratigraphy with other mollusc stratigraphies from Burgundy, the closet site to the studied region. Multivariate analysis of the malacofaunas indicates that temperature and moisture did not always vary in parallel during the Holocene. On the other hand, Verrières malacofaunas reflect the main Holocene changes, as observed in the classical pollen series, confirming the reliability of the local biostratigraphy. The younger Dryas in Verrières was cold and cry. This was followed by the Preboreal phase, which is not well preserved at Verrières, but shows cool and humid conditions. The Boreal and Subboreal both show a cold and moist event bounded by two temperature phases. The Atlantic is also divided into two temperate phases by a cool and moist event. The Subatlantic shows temperature oscillations with cool peaks, but moisture shows a continuous trend to dryness.     

Quaternary deposits in the Maltese Islands: a microcosm of environmental change in Mediterranean lands

The Quaternary deposits of the Mediterranean countries provide clear evidence of past environments, and of environmental change. Modern work enables the reinterpretation of these deposits and shows arid glacial stages and humid interglacial stages, with the Holocene characterised by considerable human impact, mostly resulting from agricultural activity. In this paper, examples of typical Mediterranean Quaternary lithofacies, including tufa and slope, fluvial, coastal, aeolian and cave deposits are described from the Maltese Islands. The deposits are correlated and their climatic signal is interpreted to provide a first modern assessment of Quaternary environmental change in the Maltese Islands.     

The Watten boring - an Early Weichselian and Holocene climatic and palaeoecological record from the French North Sea coastal plain

A deep boring (30 m), carried out at Watten (Nord, France) in 1990, has provided an opportunity to undertake palaeoecological studies based on stratigraphy, sedimentology, palynology, malacology and ¹⁴C datings on Early Weichselian and Holocene deposits. The location of the site in the inner zone of the coastal plain, extending in the Aa valley from the southernmost shores of the North Sea upstream to Watten, allows the comparison between interstadials recorded in a pure fluvial environment and the overlying Holocene sediments, in which alternation of marshy, fluvial and marine deposits are observed. The Early Weichselian sequence can be correlated with the Brørup and Odderade interstadials described in northwestern Europe, and belongs within the same palaeogeographical area. At that time the conditions were already continental at Watten. However, the last (Holocene) interglacial has been characterized, at least since the Boreal chronozone, by an oceanic climate and a progressive invasion of the sea, sometimes interrupted by stillstands and withdrawals.     

Can 234U–230Th dating be used to date large semi‐arid tufas? Challenges from a study in the Naukluft Mountains,Namibia

The large, extensive tufa deposits of the semi‐arid Naukluft Mountains, Namibia are potentially important palaeoenvironmental indicators in an area with few proxy records. Tufas are reliable indicators of increased moisture availability, and have been shown to be amenable to ²³⁴U–²³⁰Th dating, although two challenges are detrital contamination and open‐system behaviour. Densely cemented tufa facies are good candidates for dating, minimising these problems. We report attempts to date five densely‐cemented units, which are only found rarely within the Naukluft deposits. We applied a detailed methodology using multiple subsample analysis, measurement of insoluble residues, application of ‘isochron’ mixing lines, and attempted open‐systems modelling, alongside observations of micromorphology and cathodoluminescence in order to assess the validity of any obtained dates. Surprisingly, densely cemented tufas were found not always to be suitable for dating. Two units contained detrital contamination, which could not be corrected for using a single leachate correction or ‘isochron’ methods. Two units contained ‘excess ²³⁰Th’. This could result under a closed‐system if initial (²³⁴U/²³⁸U) was sufficiently high. Alternatively this may be the result of open‐system behaviour, and loss of uranium, or incorporation of initial unsupported ²³⁰Th, which render samples unsuitable for ²³⁴U–²³⁰Th dating. Micromorphological appearance and cathodoluminescence behaviour are used to explore these possibilities. This study exemplifies the need for careful sample selection, and highlights the importance of analysing multiple subsamples from any tufa sample. The detailed methodology applied proves to be a powerful tool for identifying the range of problems that can be encountered when selecting suitable candidate samples for successful dating. It also shows that semi‐arid tufa sequences may contain very little material suitable for dating. A reliable age of c 80 ka was obtained for a banded unit within a large fluvial barrage, with less reliable dates suggesting tufa deposition during times since >350 ka through to the late Holocene. Copyright © 2010 John Wiley & Sons, Ltd.